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An assessment of mineral composition can be made from spectrophotometric observations, and plausible densities and masses can then be assigned to well-observed small bodies ().

Cratering rates are estimated from the collision rates and from the masses and impact velocities of the colliding bodies by means of either empirical crater scaling laws or by more elaborate computer calculation of crater formation (Shoemaker, 1977).] Significant uncertainties are associated with each of these steps, particularly with the assignment of masses and with the calculation of crater sizes.

In the case of asteroidal bodies that collide with Earth, it has been shown that these bodies are closely related to asteroidal objects that impact the other terrestrial planets.

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If the rate at which craters are formed is known, then it is possible to estimate the absolute age of the surface.

The present rate of crater formation can be estimated from telescopic observations of various planet-crossing objects.

The Earth-Moon system also provides the essential record needed to determine the past variation of this cratering rate (Hartmann, 1972a).

If the cratering history is known for one planet or planet-satellite system, then, in principle, it can be derived for other planets and satellites, provided that the bodies impacting the various planets and satellites are dynamically related.

Although it is not always possible to date a geologic event or surface on an absolute time scale, it may be possible to establish the order in which events occurred by the traditional methods of superposition and cross-cutting relationship among various geologic units.

Material units that were deposited on other units clearly postdate the units on which they lie.

Absolute age dating determines the "calendar" time at which a rock, surface, or feature formed; relative age dating determines the order-but not the time-of formation. If the rocks have remained as closed isotopic systems, it is possible to calculate their age by measuring the amount of radiogenic isotopes relative to the amount of stable isotopes now present.

In practice, this procedure requires an accurate assessment of the initial abundances of the isotopes produced in the radioactive decay.

The technique depends on an erosion model that relates the shape of a crater to the integrated flux of meteoroids and secondary debris that have impacted the surface since the crater was fresh.

The method provides a means of estimating absolute surface ages in areas not sampled by the Apollo missions and suggests that some mare regions may be as young as about two billion years.

To establish a surface history, it is necessary to determine the sequence of various geologic events and, if possible, their duration.

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